1. Technical Field
The present invention relates to a maintenance method of a radiological image detection apparatus.
2. Related Art
In recent years, radiological image detection apparatuses employing an FPD (Flat Panel Detector) for detecting a radiation image and generating digital image data are utilized in practice, and spreading rapidly because the image can be checked immediately in comparison with the case of a conventional imaging plate. Such radiological image detection apparatuses of various types are proposed. An example of this is an indirect conversion type.
A radiological image detection apparatus of indirect conversion type has: a scintillator formed from fluorescent material such as CsI (cesium iodide) for emitting fluorescence in response to radiation exposure; and a sensor panel constructed such that a plurality of photoelectric conversion elements and switching devices of thin-film type are arranged in two dimensions on a substrate. Radiation transmitted through an image-taking object is converted into light by a scintillator in the radiation image conversion panel. Then, the fluorescence from the scintillator is converted into electric signals by the photoelectric conversion elements in the sensor panel.
Further, a radiological image detection apparatus of so-called front face reading (ISS: Irradiation Side Sampling) type is also proposed that is obtained by modifying a radiological image detection apparatus of indirect conversion type such that radiation enters from the sensor panel side (for example, see Patent Document 1 (JP-A-7-27864)). According to this radiological image detection apparatus, the intensity of fluorescence of the scintillator emitted near the sensor panel increases and hence sensitivity is improved. This reduces the amount of exposure necessary for detection of a radiation image, and hence reduces the amount of exposure in an image-taking object.
Further, a technique is also proposed that a scintillator is constructed in the form of a group of columnar crystals made of fluorescent material such as CsI so as to improving the sensitivity of the radiological image detection apparatus (for example, see Patent Document 2 (JP-A-2011-017683)). Columnar crystals formed by gas-phase deposition do not include impurities such as binder, and further provides a lightguide effect of guiding the emitted fluorescence along the direction of crystal growth and hence suppresses diffusion of the fluorescence. This improves the sensitivity of the radiological image detection apparatus and improves the sharpness of the image.
Here, in the radiological image detection apparatus of ISS type, radiation is transmitted through the substrate of the sensor panel and then enters the scintillator. The substrate of the sensor panel is typically composed of glass. Nevertheless, glass absorbs radiation in not a little amount. This causes a concern of attenuation of the radiation entering the scintillator. Thus, in the radiological image detection apparatus described in Patent Document 1, the substrate of the sensor panel is composed of a resin sheet having a lower radiation absorbing power than the glass. Alternatively, even when glass is employed, a glass sheet as thin as a few 100 μm or the like is adopted.
The characteristics of radiological image detection apparatuses of the above type may vary as they deteriorate with age, and a modulation transfer function (MTF) and a noise power spectrum, for example, are employed as evaluation items of deteriorations with age (refer to Patent document 3 (JP-T-2004-518958(The symbol “JP-T” as used herein means a published Japanese translation of a PCT patent application.)), for example).
In ISS radiological image detection apparatuses, the attenuation of radiation entering the scintillator is further reduced if the substrate of the sensor panel is removed. However, if the substrate is peeled off, the moisture resistance which is attained by the substrate is lost. As a result, moisture permeates into the sensor thin film in which the photoelectric conversion elements and the switch elements are formed and may even permeate into the scintillator through the sensor thin film. Since CsI which is the material of the scintillator exhibits deliquescence, the scintillator may deteriorate due to the moisture permeation.
Where the substrate exists, the deterioration of the scintillator due to moisture permeation advances from its peripheral portion. On the other hand, if the substrate is peeled off, the moisture permeation occurs over the entire area of the scintillator and the scintillator may deteriorate from its central portion. Since the central portion of the scintillator is mainly used for radiation image detection, to maintain high accuracy of diagnoses that are based on radiation images, it is necessary to detect deterioration of the scintillator properly.
In the maintenance method disclosed in Patent document 3, deterioration of a detection apparatus is detected by monitoring a variation with age of NPS which is the sum of various noises such as a structural noise of a scintillator, an X-ray quantum noise, and an electrical noise of a sensor panel. Whereas deterioration of a scintillator appears as the structural noise, the contribution of the structural noise to NPS is relatively small. Therefore, it is difficult for NPS evaluation to detect deterioration of a scintillator properly. Furthermore, to extract only a structure noise component from NPS, it is necessary to perform a complex analysis on many images that are acquired under such various sets of conditions that individual noise components can be discriminated from each other. As such, extraction of only a structure noise component is difficult.